Compressors including an impeller, such as a centrifugal compressor, have advantages such as having better efficiency, being smaller in dimensions and in weight and being more stable in operation than reciprocating compressors, but have a limited range of operating conditions relating to the flow rate. Under an operating condition at a low flow rate, a centrifugal compressor generates a phenomenon such as considerable fluid separation at the internal flow field, and causes a phenomenon of unstable operations. This causes stall and accordingly surge, thus rapidly decreasing the efficiency and the pressure-ratio of the compressor, shortening the life of the compressor and accordingly causing a damage of the compressor in a short time.
A vaneless diffuser of a centrifugal compressor has a flow channel, on both sides of which an annular cap and an annular disk are provided in a fixed manner, where their shapes are determined depending on the operating condition at the design point. This enables the most excellent performance at the design point, whereby the kinetic energy of fluid at the outlet of the impeller can be converted effectively into static-pressure energy. Conventional vaneless diffusers are structured axisymmetrically. That is, such a vaneless diffuser has a width that is uniformly distributed in the circumferential direction. During the operation at a low flow rate, the vaneless diffuser generates considerable fluid separation inside it, and such a stall phenomenon increases flow loss and so decreases the efficiency of the diffuser. As the flow rate further decreases, the kinetic energy of the fluid in the radial direction is not sufficient, and so the fluid flows backward due to the action of adverse pressure gradient and a surge phenomenon occurs at the compressor.
Herein the term vaneless in a vaneless diffuser refers to a diffuser as a flow channel that is not provided with vanes (blades).
Conventionally known methods of suppressing the stall in a vaneless diffuser provide a diffuser having a decreased width so as to increase the kinetic energy of the fluid in the radial direction for a low flow rate and to decrease the adverse current.